Section 17 provides that local authorities must "safeguard and promote the welfare of children within their area who are in need", where 'in need' is defined thus:
"For the purposes of this Part a child shall be taken to be in need if—
(a) he is unlikely to achieve or maintain, or to have the opportunity of achieving or maintaining, a reasonable standard of health or development without the provision for him of services by a local authority under this Part;
(b) his health or development is likely to be significantly impaired, or further impaired, without the provision for him of such services; or
(c) he is disabled"
So the scope of that section is wider than just 'disability'. It could, for example, include a child who is living in a house with severe damp and mould problems or with inadequate sanitation (leading to potential health problems). It could also include a child whose mental development was likely to be impaired through him never being allowed to leave a single room in his home.
http://www.legislation.gov.uk/ukpga/1989/41/section/17
Section 47 requires a local authority to investigate whether it is necessary for them to take action in respect of a child who "is the subject of an emergency protection order or . . . is in police protection" or where they have reasonable cause to suspect that a child . . . is suffering, or is likely to suffer, significant harm".
http://www.legislation.gov.uk/ukpga/1989/41/section/47